Field of the Invention
The present invention relates to a device for inductively heating a heating element via a magnetic field generated by an induction coil, having an induction coil connected to a resonant circuit, and to a method for determining a temperature of a heating element.
Description of the Background Art
Electrically conductive materials can be heated by induction. This occurs by placing an electrically conductive material in a magnetic field generated by an induction coil. The magnetic field is hereby generated by an alternating current, which results in a polarity reversal of the magnetic field at the frequency of the alternating current.
Eddy currents are induced in the electrically conductive material by the alternating magnetic field. These induced alternating currents work against the specific resistance of the material, as a result of which heat is produced.
The induction can occur in this case through nonconductive materials, which experience no heating. Only the radiation of heat from the electrically conductive material can lead to a heating of the surrounding nonconductive materials.
Heating by induction can be found in many applications today. The most frequent industrial uses are, for instance, the tempering, annealing, melting, or welding of metals. But induction heating can be found in household appliances as well, for instance, in induction cooktops.
Induction heating is used furthermore also for heating fluids that flow around a heating element. Induction heating is especially suitable for use in water circulation systems in electric vehicles, because electrical energy can be converted to heat with a relatively high efficiency. This is especially advantageous, because in electric vehicles no waste heat arises from the internal combustion engine and therefore cannot be utilized for heating, for instance, the passenger compartment.
In order to be able to regulate selectively the emitted heat output of an induction heating system and to be able to ensure that the induction heating system is not overheated, it is necessary to be able to accurately determine the temperature of the heating element being heated inductively. Various methods are known from the prior art.
For example, the temperature of an object can be acquired by temperature sensors. These can be placed either directly on the object or be attached to the object in conjunction with a thermal bridge. These temperature sensors work, for instance, by the principle of the temperature-dependent change in the resistance of the sensor. To this end, however, the sensor or the thermal bridge would be used as an additional part, incurring costs and taking up additional installation space.
In addition, also known are optical temperature measuring devices that determine a temperature contactless by means of optical methods. In order to be able to use optical methods, the region to be measured must be visible and in the best case also be accessible. This is not possible everywhere, however.
The determination of the temperature of a heating element heated by induction also occurs, moreover, by methods utilizing the temperature-dependent permeability properties of a material. This is disclosed in, for example, DE 42 38 862 C2, which corresponds to U.S. Pat. No. 5,255,981.
It is a particular disadvantage of this method that it cannot be used for materials that have a constant permeability for the temperature range of interest in the specific application, as a result of which the material selection is limited, or the method can be used only in special material/temperature range combinations.
In the case of all methods in the prior art, additional components are required for performing a temperature measurement or the to be measured site provides sufficient access.